Pheochromocytomas and paragangliomas are uncommon tumors that are exceedingly dangerous. They synthesize and release epinephrine and/or norepinephrine. We continue efforts to improve diagnostic accuracy and to develop better means for treating these patients. We have recently developed a new technique for measuring plasma concentrations of metanephrines, using HPLC and electrochemical detection. Application of this technique, as well as measurements of plasma catecholamines and urinary metanephrines, were applied to 42 patients with pheochromocytoma, 34 patients with essential hypertension, and 44 normotensive control subjects. The presence of a pheochromocytoma resulted in considerable larger increases in plasma metanephrines than in catecholamines. The test of plasma metanephrines in patients with pheochromocytoma has 100% sensitivity. If the diagnosis were based on plasma catecholamines, there were six false negative results, yielding 86% sensitivity. If urinary metanephrines were the basis for the diagnosis, there were three false negatives, or 92% sensitivity. False positive rates were 12% for both plasma metanephrines and catecholamines. Thus, measurement of plasma metanephrines provides the most sensitive single clinical test yet developed for the diagnosis of pheochromytoma.